How did her question make you feel?

I think it is a legitimate question from her. Especially if it is in the context of the discussion - if you maybe talked about other times you've felt this way about someone else, for example.

Like any good 'parent' she must take care of herself first to be available to you. Asking this question was possibly a way to show she has this boundary.

Keep talking about it. Next session, please talk about how you feel about her asking this, even though I imagine it must be very difficult. Working through this with her means you won't have to find another therapist and begin again, and have to work through it there. Now's the time